The present invention relates to a method for radio transmission of digital signals, as well as, a system for radio transmission of digital signals.
A digital telecommunications system using burst transmission and code division multiplex has already been disclosed in German reference DE 43 29 317 A1. Digital mobile radio may be regarded, in particular, as an application for such a telecommunications system. In mobile radio systems, a multiplicity of mobile subscribers access the radio channel transmission medium. The multiple access problem resulting from this can be solved using the elementary multiple access methods "frequency division multiplex" (Frequency Division Multiple Access, FDMA), "time division multiplex" (Time Division Multiple Access, TDMA) or "code division multiplex" (Code Division Multiple Access, CDMA), or using combinations of these methods. In CDMA mobile radio systems, a plurality of subscribers transmit in the same frequency band at the same time. For this reason, the entire received signal is governed by the elements from a plurality of subscribers. The various subscriber signals can be separated, and the data transmitted by the individual subscribers determined, by the use of optimal estimation algorithms. In order that the said estimation algorithms can be used, it is necessary for the channel pulse responses of the radio channels of the individual subscribers to be known. As a rule, it is made possible for the receiver to obtain this information by overlaying into the transmitted signals signal sections which have been agreed in advance with the receiver and are thus known in the receiver, so that the receiver determines the channel pulse responses from the response of the channel to these known signal sections. The overlaid signal sections for determining the channel pulse responses are called training sequences. It is assumed that the signal transmission takes place in blocks, a transmitted block being called a burst. Such a burst is in each case composed, for example, of a training sequence in the form of a specific interblock sequence for channel estimation and of two data blocks which surround this interblock sequence. The burst can also be composed of in each case one training sequence in the form of a specific preamble for channel estimation and a data block which follows this preamble. Alternatively, a burst may in each case be composed, for example, of a training sequence in the form of a specific postscript to the channel estimation and of a data block which precedes this postscript. The training sequence can also be of any other nature within a burst.
Symbol interference can occur in the received signal as a result of components of the training sequences, that is to say there are values of the received signal which are governed both by data symbols and by the training sequences. This interference which is caused by training sequence components is produced, for example, when using an interblock sequence as the training sequence at the end of the data block which is in each case transmitted in a burst before the interblock sequence, and at the start of the data block which is in each case transmitted in a burst after the interblock sequence. This interference leads to a coded bit error rate of more than 10.sup.-2 in the subsequent data detection, even when a high signal-to-noise ratio is present.